In Sweet Home 3D, you can lock all walls or unlock them, but you can’t lock individual walls.

I don’t know Autocad so the only thing that "straight" walls makes me think about in Sweet Home 3D context, is its magnetism that you may enable or disable in the preferences to help you draw walls at an angle multiple of 15º or at any angle. See help about magnetism for more information.

I’m not sure to fully understand what you mean by "trimming". Did you try the recently added menu item Plan > Join walls? Is it what you look for?